The correct answer is C. Blood flow can increase as much as 20-fold in exercising skeletal muscle, which is a greater increase than in any other tissue in the body. This tremendous increase in blood flow results almost entirely from the actions of local vasodilator substances on the muscle arterioles. During exercise, the muscles use oxygen more rapidly than it can be delivered by the blood, which decreases the oxygen concentration (choice D) in the tissues. The oxygen deficiency causes vasodilator metabolites (choice B) such as adenosine, carbon dioxide, lactic acid, and others to accumulate in the tissues. The vasodilator metabolites acting on the arterioles lead to a reduction in vascular resistance (choice E) and an increase in blood flow (choice A).

The correct answer is C. The same volume of blood flows through each of the different types of blood vessels each minute. Because the capillaries have the largest cross-sectional area (averaging 2500-5000 cm²), and because the velocity of blood flow is inversely related to cross-sectional area, it is clear that the mean linear velocity of a red blood cell is lowest in the capillaries. Under resting conditions, the mean linear velocity of a red blood cell in the capillaries is 0.3-0.6 mm/sec, whereas, the velocity in the aorta (choice A) is about 200 mm/sec. This low velocity of red blood cells in the capillary network allows plenty of time for oxygen to diffuse to the tissues.

The velocity of blood flow is ranked from highest to lowest as follows: aorta (choice A) > vena cavae (choice E) > large veins (choice E) > small arteries (choice D) > arterioles (choice B) > small veins (choice F) > venules (choice F) > capillaries. This ranking assumes the vena cavae have a larger cross-sectional area than the aorta; however, when the vena cavae are partially collapsed (which occurs often) they have a lower cross-sectional area and a higher velocity of blood flow compared to the aorta.

The correct answer is B. The glossopharyngeal nerve (CN IX) and the vagus nerve (CN X) carry afferent information to the medulla from the carotid sinus and aortic arch baroreceptors, respectively. The firing rate of these neurons increases with increasing blood pressure. Therefore, by artificially increasing the firing rate of these nerves, the medulla receives a false signal that indicates that the blood pressure is too high. This elicits a baroreceptor reflex, resulting in a decrease in sympathetic outflow and an increase in parasympathetic outflow, which leads to bradycardia and hypotension.

The correct answer is A. The ductus arteriosus connects the left pulmonary artery to the aortic arch. It is derived from the left sixth aortic arch. During prenatal life, the pressure gradient causes blood to flow from the left pulmonary artery to the aorta. However, after birth, the pressure gradient reverses, and if the ductus arteriosus remains patent, the flow is from the aorta to the left pulmonary artery.

The ductus arteriosus does not connect to the pulmonary veins or the right pulmonary artery (choices B, C, and E).

The flow through the ductus arteriosus is from the left pulmonary artery to the aorta (choice D) prior to birth, but reverses after birth.

The correct answer is B. Total peripheral resistance (TPR) is equal to the pressure gradient across the circulation (mean arterial pressure - right atrial pressure) divided by the cardiac output. Right atrial pressure is assumed to equal 0 mm Hg. Thus, TPR = 100/4000 = 0.025 mm Hg/mL/min. The "ABC rule" is useful in remembering the relation between pressure (P), flow (Q), and resistance (R) because P=QR (in alphabetical order). Note that choices A, C, D, and E can be eliminated quickly because in each case the units are incorrect.

The correct answer is D. The glossopharyngeal nerve (CN IX) and the vagus nerve (CN X) carry afferent information to the medulla from the carotid sinus and aortic arch baroreceptors, respectively. The firing rate of these neurons increases with increasing blood pressure. Therefore, severing these nerves sends the medulla a false signal that the patient has suddenly lost all blood pressure. This elicits a baroreceptor reflex, resulting in an increase in sympathetic outflow and leading to tachycardia and hypertension.

The correct answer is D. The baroreceptor mechanism is important for maintaining arterial pressure when a person sits or stands from a lying position. When a person suddenly stands, the blood pressure in the brain and upper body tends to fall, which initiates a strong sympathetic discharge throughout the body aimed at returning blood pressure to normal. Increasing sympathetic stimulation to the heart causes an increase in heart rate, conduction velocity, and myocardial contractility (compare with choice A). The sympathetic stimulation also causes constriction of nearly all the arterioles in the body, which greatly increases the total peripheral resistance (compare with choice B). Sympathetic stimulation of the renal vasculature leads to a decrease in renal blood flow (compare with choice E). Constriction of large veins (compare with choice C) increases venous return to the heart, causing the heart to pump increased amounts of blood.

The correct answer is B. As blood flows through the systemic circulation the mean pressure of the blood decreases from about 100 mm Hg in the aorta to about 0 mm Hg in the right atrium. The mean blood pressure is about the same in all portions of the aorta (choice A) and it only falls by a few mm Hg in the large arteries (choice A). The blood pressure decreases by 10 to 20 mm Hg in the small arteries (choice D) so that blood entering the arterioles has a pressure averaging about 80 to 90 mm Hg. By the time the blood has reached the ends of the arterioles (choice B) the pressure has fallen to about 35 mm Hg. The pressure falls another 25 mm Hg as it flows through the capillary network (choice C), so that blood entering the venules has a pressure of about 10 mm Hg. The blood pressure falls by about 10 mm Hg as it flows along the venous system (choices E and F) to the right atrium.

The fall in blood pressure (from 100 to 0 mm Hg) along the various types of blood vessels in the circulation is summarized in the table. The table also shows the relative resistance (expressed as % of total peripheral resistance) to blood flow in the various segments of the circulation.

The correct answer is D. The fibrous pericardium, which surrounds the heart, does not simply separate the heart from other chest structures, but has the important physiologic role of limiting the distension of the heart during diastole. This helps keep the (normal) heart functioning in a useful part of Starling's curve. In congestive heart failure, the slow enlargement of the heart also enlarges the fibrous pericardium, and this protective function may be lost.

The lungs (choice A) and diaphragm (choice C) do not usually significantly limit cardiac expansion during diastole.

Shutting and opening of the aortic (choice B) and mitral valves (choice E) are mechanical events that occur secondary to the changes in muscle tone in the cardiac chambers.

The correct answer is A. A decrease in arterial compliance indicates that the arterial wall is stiffer (i.e., less distensible). When the compliance of the arterial system decreases, the rise in arterial pressure becomes greater for a given stroke volume pumped into the arteries. In the normal young adult, the systolic blood pressure is about 120 mm Hg and the diastolic blood pressure is about 80 mm Hg. Because the pulse pressure is the difference between the systolic and diastolic blood pressures, the normal pulse pressure is about 40 mm Hg in a healthy young adult. However, in older adults the pulse pressure sometimes increases as much as two times normal because the arteries become hardened by arteriosclerosis.

The cardiac output (choice B) itself has no direct effect on the pulse pressure; however, if a decrease in cardiac output is associated with a decrease in stroke volume, the pulse pressure would be expected to decrease.

A decrease in myocardial contractility (choice C) would be expected to decrease stroke volume, and therefore cause the pulse pressure to decrease.

A decrease in stroke volume (choice D) causes the pulse pressure to decrease because a smaller amount of blood enters the arterial system with each heartbeat, and the rise and fall of pressure during systole and diastole is decreased.

A decrease in total peripheral resistance (choice E), i.e., vasodilation, does not have a significant effect on the pulse pressure of the major arteries under normal conditions.

The correct answer is C. Velocity has increased 7-fold, indicating a decrease in cross-sectional area by a factor of 7. This would be caused by an obstruction, not an aneurysm.

Choice A is incorrect, because a coronary artery aneurysm would produce an increase in cross-sectional area rather than a decrease.

Flow has increased 7-fold, indicating a decrease in vessel diameter, thus choices B and D are incorrect.

The correct answer is C. The table shows that the greatest fall in blood pressure (50 mm Hg) occurs in the arterioles, which indicates that the arterioles account for about 50% of the total peripheral resistance. The structural characteristics of arterioles are consistent with their function as control valves that regulate blood flow to the capillary networks of the body. Thus, arterioles are thick-walled vessels with the highest ratio of wall cross-sectional area to lumen cross-sectional area. This does not mean that arterioles have thicker walls compared to arteries. It simply means that the walls of arterioles are relatively thick compared to their overall size (diameter). The wall-to-lumen ratio of arteries, which includes the aorta (choice A) as well as large (choice A) and small arteries (choice B), is less than that of arterioles but greater than that of venules and veins (choices E and F). The capillaries (choice D) lack smooth muscle cells in their walls, which makes wall-to-lumen ratio measurements much less meaningful.

The correct answer is C. The glossopharyngeal nerve (CN IX) and the vagus nerve (CN X) carry afferent information to the medulla from the carotid sinus and aortic arch baroreceptors, respectively. The firing rate of these neurons increases with increasing blood pressure. Therefore, stimulation of the glossopharyngeal nerve sends the medulla a false signal that the animal has suddenly had an increase in blood pressure. This elicits a baroreceptor reflex resulting in a decrease in sympathetic outflow and an increase in parasympathetic outflow, leading to hypotension and bradycardia.

The correct answer is A. When the balloon is deflated, the catheter simply measures the pulmonary artery pressure (choice D), which is pulsatile with systolic/diastolic values of 25/8 mm Hg. When the balloon is inflated, the catheter is "wedged" in a small branch of the pulmonary artery and the pressure that is measured is called the "pulmonary wedge pressure." Because inflation of the balloon obstructs all blood flow in the artery branch, the blood vessels distal to the point of obstruction also have no flow. One can think of these distal vessels as physical extensions of the catheter, as they allow blood pressure to be measured on the other side of the pulmonary circulation, i.e., in the left atrium. The pulmonary wedge pressure is usually a few mm Hg higher compared to the left atrial pressure, but the general opinion is that pulmonary wedge pressure is a reflection of events in the left atrium. It is usually not feasible to measure left atrial pressure directly in the normal human being because it is difficult to pass a catheter retrograde through the aorta and left ventricle. Therefore, the pulmonary wedge pressure provides an important clinical estimate of left atrial pressure. Be aware that pulmonary wedge pressure may also be called pulmonary capillary wedge pressure, pulmonary arterial wedge pressure, or simply wedge pressure.

In many instances, the pulmonary wedge pressure can provide a reasonable estimate of left ventricular end diastolic pressure (choice B). However, a notable exception is during mitral stenosis, in which the pressure in the left atrium (and therefore, the pulmonary wedge pressure) is much higher than the left ventricular end diastolic pressure because of the high resistance to blood flow through the stenosed valve.

The left ventricular peak systolic pressure (choice C) occurs when the mitral valve is closed, making it impossible to be approximated using a catheter in the pulmonary artery.

The right atrial pressure (choice E) cannot be measured or approximated from a catheter in the pulmonary artery.

The correct answer is C. The cardiac output (CO) is equal to the volume of blood ejected from the heart during each systole (i.e., the stroke volume; SV) multiplied by the number of times the heart beats each minute (heart rate; HR). In other words, CO = SV x HR. Therefore, SV = CO/HR, and since CO = 5000 mL/min, and HR = 50/min, SV = 5000/50 = 100 mL.

The correct answer is D. To calculate the direction and driving force for fluid movement, use the Starling equation [net filtration pressure = (P_c– P_i) – (π_c - π_i)]. The net pressure in this case is positive, because fluid is being forced out of the capillary.

P_c = capillary hydrostatic pressure = 20 mm Hg, π_c = capillary colloid osmotic pressure = 15 mm Hg, P_i = interstitial hydrostatic pressure = 6 mm Hg, and π_i = interstitial osmotic pressure = 7 mm Hg. Substituting these values into the equation and solving for the net filtration pressure, we get:

Net filtration pressure = (20 – 6) – (15 – 7) mm Hg, or

Net filtration pressure = 14 – 8 = 6 mm Hg

Note: You can immediately narrow the answer options to D, E, or F because the fluid is leaving the capillaries, so the answer must be positive.

The correct answer is G. The total blood volume of the body is about 5000 mL. The systemic veins contain about 64% of this volume or about 3200 mL. The vena cavae (choice F) contain a small fraction of the total venous volume. No other segment of the circulation comes close to the amount of blood contained by the systemic veins: the pulmonary vasculature (choice E) contains about 450 mL; the chambers of the heart (choice D) contain about 350 mL; the aorta and large arteries (choice A) together contain about 650 mL; and the arterioles and capillaries (choices B and C) together contain about 350 mL. Although the capillaries contain less than 7% of the total blood volume, they have a very large surface area which facilitates diffusion exchange of nutrients and metabolites between the blood and tissue spaces.